1. Field of the Invention
The present invention relates to a positioning system, such as may be used to position a moveable object table in three degrees of freedom. More particularly, the invention relates to the use of the positioning system in a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second, movable object table for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Description of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single substrate will contain a whole network of target portions which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In general, apparatus of this type contained a single first object (mask) table and a single second object (substrate) table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughout, which in turn improves the cost of ownership of the machine.
In a known lithographic apparatus, the drive unit of the positioning mechanism for the substrate table comprises two linear Y-motors each of which comprises a stator which extends parallel to the Y-direction and is secured to a base of the positioning mechanism, and a translator (Y-slide) which can be moved along the stator. The base is secured to the frame of the lithographic device. The drive unit further comprises a linear X-motor which includes a stator which extends parallel to the X-direction, and a translator (X-slide) which can be moved along the stator. The stator is mounted on an X-beam which is secured, near its respective ends, to the translators of the linear Y-motors. The arrangement is therefore H-shaped, with the two Y-motors forming the xe2x80x9cuprightsxe2x80x9d and the X-motor forming the xe2x80x9ccross-piecexe2x80x9d, and this arrangement is often referred to as an H-drive or gantry. U.S. Pat. No. 4,655,594 describes such an arrangement using hydraulic linear motors and mentions the possibility of using electric linear motors.
The driven object, in this case the substrate table, is provided with a so-called air foot. The air foot comprises a gas bearing by means of which the substrate table is supported so as to be movable over a guide surface of the base extending at right angles to the Z-direction.
To enable such an H-drive to actively control the yaw (rotation about the Z-axis) of the driven object, the two linear Y-motors are driven independently and the X-beam is usually mounted to the Y-translators by pivots (though U.S. Pat. No. 4,655,594 suggests that a rigid joint can be used). However, in this arrangement, very high loads are experienced at the pivots between the X-beam and the Y-slides. The pivots have to carry not only thrust reactions from the X-motor through the side bearings to the surrounding structure, but also the Y-motor actuation forces.
This places very high demands on the elastic hinges commonly used for such pivots, especially when the yaw motion range is relatively large.
Further problems are encountered as the pivots on the X-beam cannot always be positioned on the line of force for the Y-motors, so that the side thrust bearing of the Y-slide has to accommodate both the X-reaction forces as well as the moment created by the Y-actuator forces and the offset between the pivots and the Y line of force. The resulting high loads in the known arrangement therefore leads to a design which can be cumbersome and heavy.
An object of the present invention is to provide an improved positioning apparatus which avoids or alleviates the problems of known positioning apparatus.
According to the present invention there is provided a lithographic projection apparatus for imaging of a mask pattern in a mask onto a substrate provided with a radiation sensitive layer, the apparatus including
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate;
a projection system for imaging irradiated portions of the mask onto target portions of the substrate;
a positioning system for positioning at least one of said object tables in a plane, said positioning system comprising:
first and second generally parallel side-beams having respective first and second sliders mounted thereon;
first and second motor means for moving said first and second sliders longitudinally of their respective side beams;
a cross-beam mounted near first and second ends thereof to said first and second sliders respectively and having a third slider mounted thereon, said cross-beam and said first and second sliders being mounted together so as to form a body that is substantially rigid in translation in said plane and in rotation about an axis normal to said plane;
third motor means for moving said third slider longitudinally of said cross-beam, said third slider having an object holder for holding said one object table; characterized by:
a thrust bearing pivotally mounted to said first slider for transmitting forces in said plane and perpendicular to said first side beam between said cross-beam and said first side beam.
By mounting the cross-beam (X-beam) and first and second (Y-) sliders rigidly against rotation about an axis (the Z-axis) normal to the plane of movement of the moveable object (XY-plane) as well as against translations in that plane, the X-beam and Y-sliders form a rigid body in the XY-plane. This eliminates the need for pivots capable of transmitting the Y actuation forces and X reaction forces to and from the X-beam and simplifies the construction of the apparatus.
Further, the thrust bearing pivotally mounted between the slider and side beam thereby transfers forces in the (nominal) X-direction to the side beam without experiencing any forces in the Y-direction, simplifying the construction of the bearing and pivot. There is also no xe2x80x9ccross talkxe2x80x9d between X and Y forces; the transfer of the X-direction forces does not give rise to any Y-direction forces.
The motors driving the first and second (Y-) sliders may be linear motors having a stator mounted on the beam and an armature in the slider. The motors may be arranged to provide substantially constant characteristics independent of the angular (yaw) position of the Y-sliders, for example by having an armature of magnets arranged in a herring-bone pattern, or the driving software or hardware may be arranged to compensate for yaw-dependent properties of the motor.
Damage protection can advantageously be provided by a yaw and/or a yaw rate sensor and cutoff arranged to cut power to the motors in the event of an excessive yaw or rate of yaw of the X-beam. Resilient buffers arranged to contact the Y-beams in the event of out-of-range yaw motions can provide additional protection.
According to a yet further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a first movable object table provided with a mask holder for holding a mask;
a second movable object table provided with a substrate holder for holding a substrate; and
a projection system for imaging irradiated portions of the mask onto target portions of the substrate; the method comprising the steps of:
providing a mask bearing a pattern to said first moveable object table;
providing a substrate provided with a radiation-sensitive layer to said second movable object table;
irradiating portions of the mask and imaging said irradiated portions of the mask onto said target portions of said substrate; characterized in that:
a positioning apparatus used to position one of said movable object tables prior to or during said steps of irradiating and imaging comprises:
first and second generally parallel side-beams having respective first and second sliders mounted thereon;
first and second motor means for moving said first and second sliders longitudinally of their respective side beams;
a cross-beam mounted near first and second ends thereof to said first and second sliders respectively and having a third slider mounted thereon, said cross-beam and said first and second sliders being mounted together so as to form a body that is substantially rigid in translation in said plane and in rotation about an axis normal to said plane;
third motor means for moving said third slider longitudinally of said cross-beam, said third slider having an object holder for holding said moveable object; characterized by:
a thrust bearing pivotally mounted to said first slider for transmitting forces in said plane and perpendicular to said first side beam between said cross-beam and said first side beam.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d or xe2x80x9cexposure areaxe2x80x9d, respectively.
In the present document, the terms radiation and projection beam are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm), EUV, X-rays, electrons and ions.